The present invention relates generally to apparatus for electrotreating a metal strip and more particularly to an electrode roll for such apparatus.
An example of electrotreating is the electrolytic plating of a surface of a metal strip, e.g., electrolytic galvanizing wherein a steel strip is plated with zinc. Other examples of electrotreating include the electrolytic cleaning or pickling of a surface of the metal strip. In all these examples the metal strip is electrically charged and constitutes one electrode in an electrolytic cell having another electrode, with electrolytic liquid between the two electrodes. An electric current flows through the electrolytic liquid between the metal strip and the other electrode, and, depending upon whether the metal strip is to be plated or cleaned, ions flow to or from a strip surface to be either deposited thereon or removed therefrom.
For example, in an electrogalvanizing operation, the metal strip is provided with a negative charge, so as to be a cathode, a metallic anode is placed adjacent the metal strip surface to be coated, and the electrolytic liquid contains zinc ions. The anode may be depletable, in which case it is composed of zinc.
It is often desirable to coat only one surface of the metal strip with zinc, and in such a case, a zinc anode is placed alongside only that surface of the metal strip which is to be coated.
A recently developed electroplating process employs a series of horizontally disposed, cylindrical rolls, each having an outer surface composed of electrically conductive material. Alternating rolls in this series are electrode rolls while the other rolls in the series are contact rolls for electrically charging the metal strip. Each roll is located totally above a bath of electrolytic liquid, and no roll is in contact with the bath. A continuous metal strip having opposed flat surfaces is wrapped sequentially around a substantial portion of each roll in the series, over and under alternate rolls. One surface of the strip touches each contact roll, and the other strip surface is in closely spaced relation to the conductive, outer surface of each electrode roll. The metal strip is advanced in a downstream direction, and the rolls are simultaneously rotated while maintaining the wrapped-around relationship between the strip and each roll.
The outer surface of each electrode roll is charged with a charge having a predetermined polarity, and the contact rolls are charged with an opposite polarity. The metal strip is charged with a polarity opposite to that of the electrode rolls as a result of the strip's contact with the contact rolls. An electrolytic liquid is introduced onto the outer surface of the electrode roll at a location at or in advance of the location where the metal strip joins the electrode roll. The electrolytic liquid is maintained in the space between (a) the electrode roll's outer surface and (b) the adjacent surface of the wrapped-around portion of the metal strip by surrounding the outer surface of each electrode roll with a concentric layer of porous mesh composed of electrically non-conductive material which prevents direct electrical contact between the adjacent strip surface and the electrode roll's outer surface. The mesh layer is typically composed of intersecting strands.
As the metal strip advances in a downstream direction, that surface of the wrapped-around portion of the metal strip adjacent an electrode roll is electrotreated at that electrode roll.
The metal strip may be wrapped around the lower portion of the electrode roll, in which case the top surface of the strip would be the adjacent surface and would undergo electrotreating; or the strip may be wrapped around the upper portion of the electrode roll, in which case the bottom surface of the strip would be the adjacent surface thereof and would undergo electrotreating. A given apparatus may employ a multiplicity of electrode rolls at some of which the top surface of the strip undergoes electrotreating and at others of which the bottom surface of the strip undergoes electrotreating.
The electroplating apparatus described above is disclosed in more detail in the prior filed, commonly-owned U.S. application Ser. No. 424,858 filed Sept. 27, 1982, entitled "Method and Apparatus for Electro-Treating a Metal Strip," William A. Carter, inventor; and the disclosure thereof is incorporated herein by reference.
In an electroplating operation, generally, and in an electrogalvanizing operation particularly, it is desirable that the anode roll be depletable, e.g., composed of zinc when used in an electrogalvanizing operation. The advantages of a depletable anode roll, generally, and of a depletable zinc anode roll in an electrogalvanizing operation, particularly, are described in the aforementioned prior filed, commonly-owned U.S. application.
In the apparatus described in the aforementioned prior filed, commonly-owned U.S. application, the layer of porous mesh material is removably secured to the outside of the depletable anode roll, as by straps surrounding the mesh layer adjacent opposite ends of the anode roll; but this has certain drawbacks. The non-conductive nature of the porous mesh material retards the depletion of cations from those parts of the outer surface of the depletable anode roll covered by the strands of the porous mesh layer. As a result, craters are formed on those parts of the outer surface of the anode roll not covered by the mesh strands, while those parts of the outer surface covered by the mesh strands define ridges around the crater.
In addition to being non-conductive, the porous mesh material is also somewhat elastic, and, in the course of the movement of the metal strip around the rotating mesh-covered anode roll, the strands of elastic mesh material are stretched in both axial and circumferential directions relative to the outside of the anode roll. As a result, the ridges around the craters may become somewhat eroded, but at a substantially slower rate than the craters. When the ridges erode, the layer of removably secured mesh material becomes loosely fitting around the anode roll; and this is undesirable as it can lead to premature mutilation of the layer of porous mesh material requiring its frequent replacement.
The aforementioned prior filed, commonly-owned U.S. application discloses an arrangement for preventing the covering layer of mesh material from becoming too loose as a result of the depletion of the anode roll. In this arrangement, the outer layer of mesh material is rolled up around the anode roll, like a window shade, as the anode roll undergoes depletion. The layer of mesh material has one free end, unsecured to the anode roll or to the remainder of the mesh layer, and straps are employed to prevent this free end from flapping. The drawback to this arrangement is its relative complexity.
Premature mutilation of the layer of mesh material can also occur as a result of the stretching or shifting movement of the mesh strands in axial or circumferential directions relative to the crater ridges.
More particularly, as the metal strip moves around the rotating anode roll, the strip exerts pressure against each mesh strand. When a mesh strand is fully-supported from below by a crater ridge, it is better able to withstand the pressure exerted against it by the metal strip than when the strand is shifted to a position in which it partially overlaps the underlying crater ridge. In the latter case, the mesh strand is not fully supported by an underlying crater ridge and is less able to withstand the pressure of the metal strip, leading to premature mutilation of the mesh layer.